Chairs of this type are important both in the office sector, in which a user can ergonomically adapt the chair to the personally preferred working position, and in the private sector in that it enables the relative position between the seating portion and back rest to be modified to make it more comfortable for relaxing.
Chairs are already known comprising an inclinable seating portion and back rest which can vary their inclination to the rest of the chair by synchronous mutual rotation at a fixed ratio, by which the angle between one part and the other is varied.
These chairs have had great success but have however proved to be susceptible to improvement with regard to the ability to change the initial angle between the seating portion and back rest, which is predetermined by the synchronism ratio of the mechanism itself. It follows that given a particular back rest inclination, the user is unable to independently modify the inclination of the seating portion or vice versa, because one part fixes the other.
To avoid this drawback, a series of devices have been proposed which are able to adjust the inclination of just one of the two components while leaving the other fixed.
In particular, EP 1192876 and EP 1946674 describe an inclination adjustment device consisting of a cam which when rotated by a lever displaces the height of one of the two chair fixing points, for example the rear point, to hence modify the chair inclination.
The drawback of this system is that only two stable positions are possible, namely the starting position and that reached after the adjustment.
Continuous adjustment solutions are also known which allow adjustment of opposing surfaces of two facing elements along a plurality of substantially stable positions.
U.S. Pat. No. 6,039,338 relates to an inclined surface device which on rotation about its rotational axis thrusts against the contact surface of the opposing element. By suitably shaping the two contact surfaces, the inclination of one element to the other can be modified.
WO2007/083186 relates to a device for adjusting the rotation of at least one movable element of the chair. Although it enables the position of the opposing surfaces to be adjusted, this element has the drawback of being unidirectional so that pulling systems have to be used to maintain the opposing elements in mutual contact. The result is a bulky and costly construction which always requires the presence of additional locking means to prevent movement reversibility.